1. Field of the Invention
The present invention relates to a virtual source/virtual destination device in an asynchronous transfer mode (ATM) network and, more particularly, to a virtual source/virtual destination device which notifies a device located upstream on a signal flow of a rate and a congestion state, in an ATM network where available bit rate (ABR) service is provided for managing a band using a resource management cell (RM cell).
2. Description of the Related Art
As the service on an ATM network, ABR service has been recommended in these days. Such recommendation is recited, for example, in "ATM Forum Traffic Management Specification version 4.0" (The ATM Forum Technical Committee, April 1996). ABR service is the service intended to guarantee not transmission quality against a data transmission delay but quality against data abandonment. In other words, the service guarantees accurate transmission of data contents even when sacrificing an outgoing cell rate. In thus characterized ABR service, an allowable cell rate (ACR), which is an outgoing cell rate at an end-system, is controlled by closed loop control using a resource management cell (RM cell).
FIG. 5 is a block diagram showing concept of a conventional ATM network associated with the above-described ABR service. With reference to FIG. 5, connected to an ATM network 101 is a source 10 and a destination 20, in which an ATM switch 102 is disposed.
For the data transmission by the source 10 to the destination 20, the source 10 first sends out a forward resource management cell (FRM cell). Upon reception of the FRM cell which has sent from the source 10 and then passed through the ATM switch 102, the destination 20 returns the FRM cell as a backward resource management cell (BRM cell) to the source 10.
At the time when the FRM cell sent from the source 10 or the BRM cell sent from the destination 20 passes through the ATM switch 102, the switch writes to the FRM cell or the BRM cell, an explicit rate (ER), a congestion indication bit (CI bit) and a no increase bit (NI bit) which are information on a congestion state at that time. ER is a rate which indicates that no congestion occurs when the ATM switch is at a rate less than or equal to this rate. CI bit indicates congestion when its value is "1" and indicates non-congestion when it is "0". NI bit indicates no rate increase when its value is "1" and rate increase allowed when it is "0". The source 10 modifies an outgoing cell rate based on the ER, the CI bit and the NI bit written on the BRM cell sent from the destination 20.
ACR transmittable from the source 10 is controlled based on the rate information and the congestion information written on a turning-around RM cell. Large distance between the source 10 and the destination 20 and a long round trip time (RTT) therefore result in increase in a congestion control loop using an RM cell.
As a result, it will take a longer time for the congestion information written on the RM cell by the ATM switch 102 to arrive at the source 10, whereby rate control performance of the source 10 is degraded. In addition, since it is not before an FRM cell sent out first from the source 10 returns to the source 10 as a BRM cell that rate control by an RM cell starts operating, no improvement is made in data transfer efficiency at the start of data transmission.
For the purpose of solving the above problems, ABR service defines a virtual source/virtual destination (VS/VD) device as recited in the above-described literature, for example. VS/VD is provided halfway in the network to virtually assume a terminal behavior for conducting RM cell termination processing. Introduction of a VS/VD device into the ABR service network results in dividing a congestion control loop using an RM cell to shorten an arrival time of congestion information at the source 10, thereby improving rate control performance by the source 10. In the following description, a unit of such a divided congestion control loop is referred to as a segment.
FIG. 6 is a block diagram showing one example of an ATM network associated with ABR service in which a congestion control loop is divided into a plurality of segments by a conventional VS/VD device. In the illustrated example, the ABR service network has a congestion control loop divided into three segments 104a, 104b and 104c by VS/VDs 103a and 103b. In the following description, the side of a source 10 is referred to as being upstream and the side of a destination 20 as being downstream.
In FIG. 6, in data transmission from the source 10 to the destination 20, an FRM cell sent out by the source 10 is terminated by the VS/VD 103a and returned to the source 10 as an BRM cell. Also between the VS/VD 103a and the VS/VD 103b and between the VS/VD 103b and the destination 20, an RM cell similarly turns around within each segment. The VS/VD congestion indication system in the illustrated example is to indicate congestion, at the time when a cell buffer in the VS/VD congests, by applying the congestion indication to an RM cell turning around in an upstream segment.
The above-described conventional VS/VD device, however, has a shortcoming that rate information and congestion information written in an RM cell is neither received nor transmitted directly over divisional segments.
In FIG. 6, when congestion occurs at a switch in the downstream segment 104c, a BRM cell which stores the congestion information is terminated by the VS/VD 103b and therefore fails to arrive at the upstream segments 104b and 104a. While the VS/VD 103b reduces an ACR trasmittable to the destination 20 from the VS/VD 103b according to the contents of the BRM cell which stores the congestion information of the downstream segment 104c, an ACR from the upstream VS/VD 103a remains unchanged, so that a cell buffer of the VS/VD 103b congests. Therefore, the VS/VD 103b, upon occurrence of congestion of a cell buffer in its device, for the first time stores congestion information in the BRM cell to notify the VS/VD 103a, causing the VS/VD 103a to reduce the ACR to the VS/VD 103b.
Furthermore, reduction of an outgoing cell rate at the VS/VD 103a to the VS/VD 103b causes congestion at a cell buffer of the VS/VD 103a similarly to the generation of congestion in the cell buffer of the VS/VD 103b as mentioned above. Then, following the same procedure, a congestion indication arrives at the source 10.
As described in the foregoing, congestion control by a conventional VS/VD device has a shortcoming that when congestion occurs at any segment on a network, buffers of all the VS/VD devices existing halfway between connections congest before a congestion indication arrives at a source to reduce an outgoing cell rate at the source.
Another shortcoming is that since congestion indication and rate information indicative of a downstream congestion state do not arrive at the source until all the buffers of the VS/VDs existing halfway between connections congest, the congestion indication and notification of the rate information to the source takes much time to delay elimination of the congestion.
One of conventional ATM communication devices providing ABR service traffic is disclosed, for example, in Japanese Patent Laying-Open (Kokai) No. Heisei 7-297843, entitled "ATM Communication Device". The literature recites an ATM communication device with dynamic bandwidth allocation (DBA) comprising an accumulation device associated with DBA traffic including an accumulation device directed to ABR traffic, an device to which an ABR bandwidth request is allocated after all the remaining DBA bandwidth allocation is satisfied, and a device which interrupts, for the purpose of transmitting DBA traffic with higher priority than the others, said ABR transmission before band allocation including the ABR transmission is fully used. While the literature refers to an end-system and a switching system capable of handling traffic of ABR service, it refers nothing about a VS/VD device. It is therefore impossible to solve the problem concerning congestion indication at an ATM network into which the above-described VS/VD device is introduced.